Articles & Case Studies

Emerson improves total chlorine monitoring of cooling water at DOW Terneuzen, The Netherlands

Posted: Wednesday 19th May 2010

New monitoring system produces faster, more reliable and accurate measurements, ensuring environmental regulations are met and maintenance costs are reduced.

Emerson Process Management’s total chlorine analyzers are being used by DOW Benelux to improve the monitoring and control of chlorine in cooling water prior to its return to the sea. The greater accuracy and speed of the new system enables easier compliance with environmental regulations. Maintenance costs have also been reduced as the amperometric sensor requires less checking and cleaning than the previously installed sensor.

The DOW Terneuzen site in The Netherlands is the second largest DOW production facility in the world with 23 plants producing plastics and chemicals. The site is located on the southern shore of the Westerschelde estuary from which it extracts sea water for cooling. To control the microbiological activity in the sea water – preventing blockages and fouling – minimal amounts of chlorine are dosed. The concentration of the chlorine must be precisely monitored and controlled before the water is returned to the estuary to ensure compliance with environmental legislation.

Emerson Process Management’s Rosemount® Analytical Model TCL sample conditioning system replaces an existing DPD analyzer system that was unreliable, had become outdated and was costly to run. The DPD analyzer relies on the measurement of color change following the injection of a chemical reagent. Because of the impurities in the water, the accuracy of the measurement was poor and also an operator had to visit the analyzer daily to either clean it or reset it.

“Measuring chlorine content in water from the Westerschelde is completely different from measuring chlorine in pool water,” explained Eric Engelen of the Process Analytical department at DOW Terneuzen. “The water is salty and contains algae, mussels, seaweed and sand. This not only influences the accuracy and reliability of the measurement, but it meant that the old sensor had to be cleaned almost daily leading to high maintenance costs.”

The Model TCL uses a reagent-based amperometric method to measure total chlorine. A potassium iodide solution injected into the sample reacts with total chlorine to produce iodine. The iodine diffuses through a membrane on the end of the sensor. An electrochemical reaction inside the sensor consumes the iodine and generates a current directly proportional to the diffusion rate. The analyzer measures the current and converts it into an equivalent total chlorine concentration. This method of measuring chlorine is ideal for measuring water containing impurities and is better suited to the application at DOW.

“Water pollution and temperature changes don’t influence the reliability of the sensor and because of the faster and more accurate and reliable measurements we are now able to spot if there is something wrong with the chlorine content much earlier,” explained Engelen. “This makes it easier for us to meet our legal requirements, which is better for the environment and prevents fines being incurred.”

The previous analyzer required a daily visit from an operator for it to be either reset or cleaned. The Model TCL doses air to assist in iodine formation; this creates a self-cleaning effect which means the sensor requires almost no additional cleaning. This leads to lower operational costs than for other sensor types. As well as the amperometric sensor, the Model TCL system includes Emerson’s Rosemount Analytical model 1056 dual input analyzer. The 1056 analyzer is very easy to install and start up and its extensive diagnostic functions make it possible to monitor the status of the sensor and prevent any unplanned downtime.

The criteria DOW uses when testing new technology is quite stringent. Only technology that has clearly demonstrated it is easier to maintain or use, improves reproducibility and reliability, resists fouling or cuts costs is adopted within the plant.

”We noticed immediately that the Model TCL’s reagents lasted for more than three months and that’s a long time,” explained Jos Baart from Cegelec, who were the contractor for the installation. “The making and replacing of the reagents was very easy and achieved in a very short time minimizing costs. What also stood out was that the initial membrane lasted during the entire four month test period.”

Emerson’s Model FCL system for the determination of free chlorine in fresh water was also tested by DOW. The Model FCL was found to offer the same reliable and maintenance free characteristics as the Model TCL. Because of this, DOW has already started to implement the fresh water system on other applications at the site in an attempt to increase the reliability of the measurements and lower operational costs.

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